Efforts will be made to identify the molecules in calf serum that stimulate resting cultures of chick embryo fibroblasts to pass through the G1 period into the S phase. Work will continue to elucidate the mechanism by which insulin preferentially stimulates the synthesis of ribosomal proteins in the resting chick embryo cells. Serum increases the incorporation of 3H-leucine by the chick embryo cells one polypeptide out of almost 1000 labeled molecules that can be separated on two-dimensional O'Farrell gels. The mechanisms of increased labeling and the role of the polypeptide during the G1 period will be explored.